1. Field of the Invention
The present invention relates to a dielectric filter, a duplexer, and a communication apparatus incorporating the same, which are used in a high-frequency circuit.
2. Description of the Related Art
Dielectric filters having both band-pass characteristics and band-stop characteristics obtained by a plurality of resonant lines disposed in a dielectric block are disclosed in (1) Japanese Unexamined Patent Publication No. 8-32313 and (2) Japanese Unexamined Patent Publication No. 8-330806. In each of the dielectric filters, the plurality of resonant lines are combline-coupled in the dielectric block to obtain band-pass characteristics, and in addition, there is provide a trap resonator to form an attenuation pole.
FIGS. 10A to 10D show an example of a duplexer using the conventional art. FIGS. 10A to 10D are projection views of the duplexer, in which FIG. 10A is a front view, FIG. 10B is a left side view, FIG. 10C is a right side view, and FIG. 10D is a top view.
In this duplexer, holes and electrodes are formed with respect to a rectangular-parallelepiped dielectric block 1. Reference numerals 2 (2a, 2b, and 2c), 3, 4, (4a, 4b, 4c, and 4d), and 5 denote resonant-line holes, inside of which inner conductors are disposed to form resonant lines. Reference numerals 7, 8, and 9 denote excitation-line holes, inside of which inner conductors are disposed to form excitation lines. Reference numerals L1, L2 to Ld shown in the figure indicate serial numbers given to the above-mentioned lines in order to be referred to in an equivalent circuit shown below.
FIG. 11 is an equivalent circuit diagram of the duplexer shown in FIG. 10. In this figure, since Z12 acts a phase circuit of .pi./2 [rad] (hereinafter indicated by omitting the rad as a unit of a phase angle), (Z1 and Z12) act as trap resonators. Z3, Z4, and Z5 act as a three-stage resonator in which they are combline-coupled in sequence. Similarly, Z7, Z8, Z9, and Za act as a four-stage resonator in which they are combline-coupled in sequence. Additionally, since Zbc acts as a .pi./2 phase circuit, (Zc and Zbc) act as trap resonators.
FIG. 12 shows the pass characteristics of the duplexer described above. In this figure, the upper graph shows the pass characteristics of a reception filter, and the lower graph shows those of a transmitting filter. In the reception filter, signals of the receiving frequency band are allowed to pass through, whereas signals of the transmitting frequency band are attenuated, and in the transmitting filter, signals of the transmitting frequency band are allowed to pass through, whereas signals of the receiving frequency band are attenuated.
However, in the dielectric filters in accordance with the conventional art described in (1) and (2), although attenuation characteristics can be obtained by a polarity generated due to the coupling circuit of combline coupling and the single trap resonator, the depth (the amount of attenuation) of the polarity obtained by the coupling circuit cannot be changed. In addition, in order to bring the position of the polarity close to a pass band, it is necessary to narrow the pitch between the resonators (the distance between the resonant-line holes). However, if it is narrowed, Qo of the resonators is deteriorated.
Furthermore, in the dielectric filter according to the conventional art, the initial-stage or final-stage resonant line of the resonant lines being combline-coupled is coupled to the excitation line to obtain an external coupling, and the trap-resonator resonant line is adjacent to the excitation line, with the result that only a single attenuation pole can be obtained by the trap resonator.